1. Field of the Invention
This invention relates to fluid-filled prosthetic implants and more particularly to filling materials for medical implants.
2. Prior Art
Medical grade Silastic.RTM. brand of silicone elastomer has a long established record of safety and bio-compatibility with clinical data extending back nearly 35 years. Numerous implantable medical products, including implantable post-cataract eye lenses, heart valves, and other devices are made from this material. Over the past 30 years the outer shells of fluid or foam-filled implantable breast prostheses have been made from silicone elastomer and such prostheses are widely used for breast reconstruction following surgical mastectomy and augmentation of hypo-plastic breast tissue.
Extensive clinical data supporting the safety of silicone elastomer for implantation has been established. Recently, it has been suggested that implanted prosthetic devices comprising an elastomeric shell enclosing a silicone gel may lead to clinical problems, most of which are putatively related to the migration of the liquid silicone gel filler through the shell into the surrounding tissue. The U.S. Food and Drug Administration (FDA) has recently restricted the use of such silicone gel-filled implants to reconstructive applications and limited silicone gel augmentations to patients subject to FDA supervision. The reason for the recent action of part of the FDA has been the uncertainty surrounding the safety of gel-filled silicone implants for breast augmentation and reconstruction. Many patients have received such implants over the past 30 years and are now experiencing anxiety regarding the safety of their existing prosthesis. Furthermore, since silicone gel is radiopaque, it may interfere with x-ray detection of tumors.
New non-gel-filled versions of such prostheses, usually saline filled, are currently available and have been for some time. Saline has the advantage of being more radiolucent than silicone gel. Some of these prostheses employ an impermeable barrier for a shell material or to underlie the shell to minimize the migration of filling fluid from the prosthesis. Shells are now also commonly textured to facilitate the ingrowth of tissue and to reduce capsular contracture and anchor the prosthesis. Existing saline-filled prostheses, although meeting FDA safety and efficacy requirements, suffer from the difficulty that many of them rupture after implantation causing difficulty for the patient as the prosthesis collapses. In addition, while texturing the outer surface of the shell to reduce capsular contracture appears to be at least partially effective, many patients still experience such contracture following implantation. Further, since water and saline are denser than breast tissues, that is, they have a specific gravity greater than breast tissue, the weight of the filler displaces surrounding tissue and stretches the elastomeric shell causing cosmetically unacceptable wrinkling. Such surface wrinkling is believed to contribute to device failure by shell rupture and subsequent collapse. Over-inflation reduces this effect but contributes to spheralization and unnatural shape and feel.
Capsular contracture has received a great deal of attention in recent years. A capsule enclosing a foreign object implanted beneath the skin is a normal response. The capsule partitions the implanted object from the body. The formation of the capsule is, in itself, not usually a problem cosmetically. In many cases, however, for reasons that are still obscure, the capsule contracts, pulling itself into a hard spherical body which compresses the implanted prosthesis. Such spherical contracture is disfiguring and the capsule must be either ruptured or removed. The problem is discussed in U.S. Pat. Nos. 4,963,150 to Brauman and 4,889,744 to Quaid. It is generally believed that texturing the outer surface of a implantable prosthesis to permit tissue ingrowth disorients the collagen fibrils in the capsule thereby reducing capsular contracture. While it appears that an open-cell or textured surface reduces the incidence of capsule contracture, the mechanism for such a reduction is not known. It is desirable to provide an implantable fluid-filled prosthesis which does not utilize radiopaque silicone-gel yet retains its form and natural, tissue-like feel. It is also desirable that the prosthesis have viscoelastic properties that are similar to surrounding tissue and further reduce capsule contracture by reducing torque and mechanical forces at the prosthesis-tissue boundary.